1. Field of the Invention
The present invention relates to an inlet pipe provided in an internal combustion engine, and a molding method of the inlet pipe.
2. Description of Related Art
An internal combustion engine includes an inlet pipe configured to introduce intake air into a combustion chamber. The inlet pipe is often molded into a curved shape in accordance with a mounting space of the internal combustion engine, and positions of other components attached to the internal combustion engine.
Published Japanese Translation of PCT Application No. 2000-508397 (JP-A-2000-508397) describes that an inlet pipe is molded by blow molding. In the blow molding, a component is generally molded in the following manner. That is, resin or the like is melted and extruded from an extruder into a tubular shape, and the resin thus extruded is sandwiched by a die. In this state, the resin is in a shape just extruded from the extruder, but air is blown inside the resin having a tubular shape so that molten resin is pushed against an inner surface of the die. The resin is then cooled down to be hardened. Thus, a tubular component having an outer shape along an inner surface shape of the die is completed. By employing such blow molding, a tubular component having a curved complicated outer shape like an inlet pipe can be molded easily.
In the meantime, in the blow molding, it is possible to accurately mold the tubular component so that its outer shape fits an inner surface of the die. However, the molding is performed by blowing the air to push the resin against the die from its inside, so it is difficult to control its wall thickness. For example, in a course of pressing the resin against the die by the air thus blown, the part of the tubular component which is stretched to a large extent has a thin wall thickness, while that part of the tubular component which is stretched to a small extent has a thick wall thickness.
Accordingly, as described in JP-A-2000-508397, in a case where an inlet pipe is manufactured by the blow molding, particularly a curved portion of the inlet pipe is configured such that: resin of that outer part of the curved portion which has a small curvature is stretched, while resin of that inner part of the curved portion which has a large curvature is stretched to a small extent. Because of this, a wall thickness of the inner part having a large curvature might be thick. Note that, as the curvature of the curved portion is larger, such an increase in the wall thickness of the inner part is shown more markedly. When the wall thickness becomes thick as such, a passage sectional area in the curved portion is decreased, so that a pressure loss might be increased. Further, when the passage sectional area is decreased, a flow speed of air flowing inside the inlet pipe is increased. As a result, the air flowing inside the inlet pipe may be separated from an inner wall surface of the curved portion, which may easily cause disturbance in its airflow. Accordingly, in a case where an inlet pipe having a curved portion is manufactured by the blow molding, a pressure loss of the inlet pipe might be increased.